This invention relates to a pyrometallurgical reprocessing of irradiated nuclear reactor fuel elements to recover purified uranium and a mixture of uranium and plutonium for use as a fresh blanket and core in a nuclear reactor. More particularly, this invention relates to a process for refining spent blanket and core fuel in a single electrorefining cell by dissolving uranium and plutonium from the spent fuel in a molten chloride salt and thereafter selectively electrolytically removing first uranium and then thereafter using molten cadmium having uranium values dissolved therein chemically to displace plutonium values from molten salt and replace those values with uranium to reestablish the original salt composition.
The disposal of radioactive waste which results from the reprocessing of irradiated nuclear power reactor fuel elements is one of the major problems facing the nuclear power industry. One approach is to solidify the radioactive waste as it comes from the reprocessing facility into a stable solid material which can be stored in the earth for a period of time sufficient for the radiation to decay to acceptable levels. However, storage times required for spent reactor fuels to achieve such levels of radioactivity are on the order of a million years. This is far longer than the geologic stability of a waste repository can be expected to be maintained. One solution is to remove the extremely long lived radioactive components such as the transuranic elements of neptunium, plutonium, americium and curium from the waste so that the remaining radioactive elements, representing the bulk of the radioactive waste, need only be stored for up to about one thousand years for the radioactivity to decay to radioactive levels of uranium used in making original fuel. It is acceptable to ensure the integrity of a repository for one thousand years. The actinides thus recovered from the waste can then be reprocessed and recycled to provide additional fuel for nuclear reactors and for isotopic power sources.
As used herein, the phrase "rare earth fission product values" includes yttrium and the lanthanide fission product elements while the phrase, "transuranic values" or TRU elements include neptunium, plutonium, americium and curium values.
Molten cadmium cathodes in combination with chloride salts have been used in processing spent fuel elements from the Integral Fast Reactor (IFR) as reported in U.S. Pat. No. 4,880,506 issued Nov. 14, 1989 to Ackerman et al. and assigned to the assignee of the present application, the disclosure of which is herein incorporated by reference.
The IFR concept is a complete, self-contained, sodium-cooled, fast reactor filled with a metallic alloy of uranium, plutonium and zirconium, and equipped with a close-coupled fuel cycle. Close-coupling of the reactor and the fuel cycle facility is achieved by locating the reactor and the reprocessing, fuel refabrication, and management efficient product waste on one site. With this arrangement, it is unnecessary to ship fuel to or from the reactor site. Fission products may be processed and stored on site for long periods of time, perhaps the entire life of the reactor, before shipment to a waste repository where ultimate disposal is required. Accordingly, it is clear that reducing the volume of waste product produced for each reprocessing is inherent in and required by the IFR concept.
A pyrometallurgical process utilizing electrorefining for purification of the core fuel has been developed to reprocess the reactor fuel. In this process, the chopped fuel rods are dissolved, or transferred by anodic solution to a solid cathode and thereafter molten cadmium is used chemically to transfer plutonium from the salt to the cadmium and replace the plutonium in the salt with uranium. The apparatus disclosed in the '506 and U.S. pat. No. 4,814,046 patents may be used to accomplish part of the process of the present invention.
In general, a low carbon steel container may be used to hold the chopped up fuel spent fuel elements or rods and a low carbon steel cathode may be used on which to deposit uranium, as will be disclosed. Subsequent to the electrotransport of the spent fuel through the electrolyte, a quantity of molten cadmium having uranium values dissolved therein is put in contact with the electrolyte so that uranium dissolved in the cadmium replaces plutonium dissolved in the salt by chemical transport. The overall result is that the uranium and plutonium values in the spent fuel are transferred electrochemically and chemically to the molten cadmium with the salt eventually regaining composition it had before the onset of the electrochemical transfer of uranium and plutonium from the anode into the salt. Thus, repeated batches of spent fuel elements can be treated without substantially altering the salt composition. During the hereinafter described process, rare earth values tend to remain in the salt and build up in concentration whereas uranium and transuranic values transfer to the electrodes.
It is known in the art how to remove the rare earth values which build up in the chloride electrolyte with reducing agents such as lithium cadmium alloys or lithium-cadmium-potassium alloys in order to isolate the rare earths in a metal matrix, all as previously disclosed in the Johnson et al. U.S. Pat. No. 4,814,046.